This invention relates to motion controlling struts or vibration arrestors and, more particularly, to a strut which may be connected between relatively movable elements such as steam pipes in a power producing station and adjacent supporting structure to limit motion between the steam pipe and the structure or any pair of relatively movable mechanical elements. The prior art discloses a diverse assortment of approaches to various motion arresting problems. In particular, many attempts have been made in the prior art to arrest the motion between piping systems in thermal power plants and between various components of artillery pieces, for example, to limit the recoil of such apparatus.
Initially, it will be understood by those skilled in the art that mechanical motion arrestors are recognized as having application in certain installations where hydraulic shock absorbing systems are unsatisfactory. Thus, for example, in nuclear electrical generating facilities where radiation levels are relatively high, hydraulic shock absorbers which are subject to leakage, particularly in the presence of high temperatures, are unsatisfactory for long term usage. Thus, if shock arrestors are positioned in a nuclear generating facility to provide rigid support for piping and other elements in the event of an earthquake or pipe rupture, these devices must be expected to remain in a standby mode, permitting thermal expansion and contraction at low velocity and acceleration rates of the piping and other elements, possibly for years, and still be suitable to arrest shocks which would occur during an earthquake. In such installations, hydraulic shock absorbers do not provide the long term stability which is required.
Most prior art dry mechanical arrestor devices include an overhauling head screw which is rotationally and axially fixed to one of the movable elements. Such lead screws are typically used to rotationally drive an axially spring centered braking device which, under normal circumstances, is centered axially between a pair of braking surfaces. These braking devices engage one of the braking surfaces as the motion parameter, typically acceleration, is increased. The primary difficulty which has been encountered with devices of this type is the fact that the entire load which is to be supported by the motion arrestor must be borne by the overhauling threaded member, even under circumstances where the braking mechanism is engaged. This constraint severely limits construction of the overhauling threaded member and also limits the load which may be supported by the motion arrestor. Some attempts which have been made to remove the overhauling threaded member from the main load bearing assembly have resulted in devices which are extremely complex in construction and utilize a spring centered braking arrangement which is itself subject to failure under mechanical stress.
The most effective dry mechanical motion arrestors which have been developed in the prior art are described in detail in U.S. Pat. No. 3,876,040 and U.S. Pat. No. 3,983,965, each of which is assigned to the assignee of the present invention. The first of these systems provides an extremely effective mechanism for monitoring acceleration of the relatively movable mechanical members and permits continued motion of the mechanical elements in the presence of extremely large forces, the motion being limited once an acceleration threshold has been reached. This mechanism, however, bears the primary load through an overhauling lead screw and is therefore limited in regard to the amount of force which can be arrested by a mechanism of predetermined size. The second of these inventions is similar in many respects to the present patent application, that device incorporating synchronized nonoverhauling threaded members which are designed to interengage without contact when axially and rotationally synchronized. Since the overall operation of that previous device is similar to the operation of the present device, the disclosure in the previously filed U.S. Pat. No. 3,983,965 is incorporated in this application by reference. One difficulty encountered with this device, however, is the requirement that the force borne by the locking strut through the nonoverhauling threads must be borne by roller bearings or ball bearings used to support one of the nonoverhauling threaded members in a fixed axial but freely rotating position within the locking strut. These bearings produce stress concentrations within the locking strut which form the weakest link in the shock arrestor structure and thus limit the ultimate strength of the locking strut.
Another difficulty encountered with this device is the fact that the spring used to sense the acceleration or velocity responds to friction in the overhauling screw-nut combination as though it were an inertia force presented by the inertia mass. This response interferes with precise calibration of the motion threshold.